1. Field of the Invention
The present invention relates to a high-frequency semiconductor device, and more particularly, to an improvement in thermal stability of a multi-finger heterojunction bipolar transistor.
2. Description of the Related Art
Miniaturization is an important factor in promoting proliferation of portable terminals such as portable cellular phones. A high-output power amplifier has become a key part of a recent portable terminal.
A heterojunction bipolar transistor (hereinafter simply called xe2x80x9cHBTxe2x80x9d) has a high current gain xcex2. A GaAs-based HBT having an emitter made of AlGaAs and a base made of GaAs is characterized by its high speed and is widely used for a high-output power amplifier of a portable cellular phone.
In order to attain a high output, HBTs must be arranged in shunt with each other, to there by form a multi-finger configuration. An HBT having such a multi-finger configuration will hereinafter be referred to as a xe2x80x9cmulti-finger HBT,xe2x80x9d and individual HBTs constituting the multi-finger HBT will be called xe2x80x9cbasic HBTs.xe2x80x9d
FIG. 8 is a plan view showing one of basic HBTs constituting a conventional high-output power amplifier.
In connection with FIG. 8, reference numeral 100 designates a basic HBT; 102 designates a collector layer; 104 designates a collector electrode; 106 designates a base layer; 108 designates a base electrode; 110 designates an emitter layer; and 112 designates an emitter electrode.
In a case where a multi-finger HBT is activated, the current gain xcex2 decreases with temperature. For this reason, in contrast with a homojunction bipolar transistor (for example, an Si bipolar transistor) having a current gain xcex2 increasing with temperature, the multi-finger HBT is resistant to thermal runaway.
However, it has already been reported that, because thermal nonuniformity arises among basic HBTs 100 within the multi-finger HBT, concentrated flow of current to a certain basic HBT 100 arises, thereby inducing a rapid change in an operating current. For instance, there is a report on failure arising for this reason (IEEE Transactions on Electronic Devices, Vol. 43, No. 2, February 1996, pp. 220 to 227).
Effective countermeasure against such a rapid change in an operating current is a reduction in thermal resistance of the basic HBT 100.
Thermal resistance "THgr"TH is defined as "THgr"TH=xcex94Tj/xcex94P, where Tj is junction temperature and P is power.
FIG. 9 is a plan view showing a basic HBT constituting another conventional high-output power amplifier.
In connection with FIG. 9, reference numeral 120 designates a basic HBT consisting of two emitters; 110a and 110b designate split emitter layers constituting an emitter layer 110 of the basic HBT 120; and 112a and 112b are split emitter electrodes constituting an emitter electrode of the basic HBT 120.
FIG. 10 is a cross-sectional view taken along line Xxe2x80x94X shown in FIG. 9.
As shown in FIG. 10, reference numeral 122 designates a GaAs substrate.
The basic HBT 120 shown in FIGS. 9 and 10 is formed from two emitters in order to reduce the thermal resistance "THgr"TH.
Two types of multi-finger HBTs are prepared: namely, a multi-finger HBT which has a single emitter 112 provided between collectors 104, as does the basic HBT 100; and a multi-finger HBT which has two emitters 112 provided between collectors 104, as does the basic HBT 120. In order to examine occurrence of thermal nonuniformity, an I-V characteristic obtained when a collector voltage Vc is changed while a base current is maintained constant is obtained as a base current parameter.
FIG. 11 is a graph showing the I-V characteristic of a multi-finger HBT using the basic HBT 100 made of a single emitter. FIG. 12 is a graph showing the I-V characteristic of a multi-finger HBT using the basic HBT 120 made of two emitters.
As shown in FIGS. 11 and 12, a gradual reduction arises in an electric current within a range of small power dissipation; i.e., a range of Vc less than 6-volts or thereabouts, because negative feedback is applied from a power source to the HBTs 100 and 120 for reasons of temperature characteristics of current gain xcex2 of the basis HBT. In terms of such a characteristic, no difference is present between the multi-finger HBT using the basic HBT 100 made of a single emitter and the multi-finger HBT using the basic HBT 120 made of two emitters.
However, within a range of large power dissipation, i.e., a range of Vc=6V or greater, the basic HBT 100 comprising a single emitter causes breakdown of a collector current Ic at a voltage Vc lower than that at which the basic HBT 120 comprising two emitters causes breakdown of the collector current Ic. In other words, the basic HBT 100 causes breakdown of the collector current Ic in a range of power dissipation lower than that in which the basic HBT 120 causes breakdown of the collector current Ic. Broken lines A1 shown in FIG. 11 designate a boundary within which concentration of electric current onto the basic HBT 100 arises under the foregoing conditions. Broken lines A2 shown in FIG. 12 designate a boundary within which concentration of electric current the basic HBT 120 arises under the foregoing conditions. The broken lines A2 shown in FIG. 12 are clearly shifted toward a higher Vc range than the broken lines Al shown in FIG. 11.
Separating an emitter to be provided between collectors into a plurality of pieces is effective for preventing occurrence of thermal nonuniformity in basic HBTs.
However, if an emitter has a plurality of parts, an area for separation must be ensured between the emitter electrodes 112a and 112b. The area of a junction between the base layer 106 and the collector layer 102 becomes large, thereby increasing the capacitance of a p-n junction. An increase in the capacitance of the p-n junction deteriorates high-frequency characteristics of a semiconductor device, thereby resulting in a decrease in gain.
Japanese Patent Application Laid-Open No. Hei. 11-102916 describes a bipolar transistor which comprises a plurality of single emitters and in which first stages of a multi-stage amplifier are connected in shunt with each other. Further, there is described proper use of a bipolar transistor of single emitter structure and a bipolar transistor of multi-emitter structure, as appropriate. However, none of the transistors correspond to a heterojunction silicon transistor.
The present invention has been conceived to overcome such a drawback and is aimed at providing a high-frequency semiconductor device having an amplifier circuit, which amplifier minimizes deterioration of a high-frequency characteristic and attains high thermal stability.
According to one aspect of the present invention, a high-frequency semiconductor device comprises a first portion of an amplifier circuit and a second portion of the amplifier circuit, which amplifies a signal output from the first portion. The first portion of an amplifier circuit is formed by means of connecting a plurality of first bipolar transistors having a hetero-junction structure in shunt with each other and is provided on a first semiconductor substrate. The second portion of the amplifier circuit is formed by means of connecting a plurality of second bipolar transistors having a hetero-junction structure in shunt with each other and being provided on a second semiconductor substrate. Further, each of the second bipolar transistors has a larger number of emitter electrodes than do the first bipolar transistor.
Accordingly, a high-frequency semiconductor device according to the present invention is advantageous in that an increase in the capacity of a p-n junction between the base layer and the collector layer of each of transistors provided in a front stage of an amplifier circuit is prevented, and that occurrence of non-uniform operations of transistors provided in a subsequent stage of an amplifier circuit having great output power can be prevented. By extension, there can be constituted a high-frequency semiconductor device which minimizes deterioration of a high-frequency characteristic and has high thermal reliability.
Other objects and advantages of the invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific embodiments are given by way of illustration only since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description.